1. Field of the Invention
The present invention relates to the fabrics used as forming media in modern papermaking machines. More particularly, a method for treating a forming fabric in order to reduce or eliminate the problem of edge curl is disclosed.
2. Description of the Prior Art
Broadly stated, the purpose of modern papermaking machines is to remove water from a stock or furnish consisting of an aqueous suspension of wood fibers and a variety of other ingredients. Generally, modern papermachines are made up of three distinct sections.
The first is the forming section, where the furnish is applied to a moving screen, traditionally referred to in the industry as a wire. The wire's screen-like construction enables water to drain readily from the furnish leaving a web of wet wood fiber on its upper surface.
At the end of the forming section, enough water has drained from the wet wood fiber for it to assume the form of a wet sheet of sufficient structural integrity to be transferred to a press fabric. The wet sheet is carried on to the press section either atop this single press fabric or sandwiched between two such fabrics. The press fabrics serve the purpose of removing further amounts of water from the wet sheet. Here, however, because the water that remains will not drain from the sheet on its own accord, the combination of fabric and wet sheet are passed together through a series of presses where water is squeezed from the sheet and accepted by the fabrics.
At the end of the press section, the wet sheet proceeds to the final stage of the papermachine, the dryer section. There, the sheet is conducted or passed around each in a series of cylinders steam-heated from within. Whatever water still remains in the sheet on reaching the dryer section is gradually driven off by evaporation upon contact with the hot cylinders. Fabrics are employed in this section as well. Here, however, they do not so much carry or conduct the sheet as serve to hold the sheet in intimate contact with the surface of each cylinder as an aid to efficient drying.
The fabrics used in each section take the form of long, continuous, endless moving belts. They are either woven in endless form or seamed into that form. Depending on the papermachine, the belts can be from 1 to 10 meters wide and of considerably longer total length.
As stated rather implicitly above, the paper manufacturing operation is continuous. In other words, furnish is continuously applied to the wire, forming a wet sheet which is transferred, in turn, to the press and dryer section, emerging finally as some form of paper product.
The fabrics used in the forming stage will be our chief concern here. The forming fabrics, or wires, play a crucial role in the papermaking process. They must be highly permeable and allow large quantities of water to drain quickly from the furnish, and must be of a weave to assure optimum sheet formation. Of equal importance, the upper surface of the forming fabric, to which the furnish is applied, should be as smooth as possible in order to assure the formation of a smooth, unmarked sheet.
Formerly, the fabrics used in the forming section were woven from metal threads. For this reason, they are still commonly referred to in the papermaking industry as wires, even though most are now woven from synthetic monofilament.
A wide variety of these fabrics are in current use, and can be characterized by weave pattern and number of layers. One chooses a particular fabric to meet the requirements of the machine on which it is to be installed and the kind of paper to be produced.
Generally, the systems of yarns in a woven fabric lie in directions which can be identified with reference to the directions they take when the fabric is in its position of use on the papermachine. The machine direction yarns lie in the direction in which the fabric as a whole moves when the machine is operating and, accordingly, must bear forces of tension associated with this motion.
Transverse to the machine direction yarns are cross-machine direction yarns. By contrast, the cross-machine direction yarns are subjected to very little, if any, tension on the papermachine.
In some of the weave patterns in current use, the cross-machine direction yarns pass over more than one machine direction yarn before weaving under one such yarn and repeating the pattern. A fabric is thereby produced having an upper surface formed primarily from the cross-machine yarns or shutes. Normally, this side is used for the formation of the paper sheet, and can be referred to as the long-shute knuckle side.
An undesired consequence of such weave patterns is that the forming fabrics so characterized tend to curl in a direction toward the long-shute knuckle side with time as back side wear and/or shrinkage occurs. The curl arises because these cross-machine weave patterns result in the shrink forces on each side of the upper surface being unequal. The resulting curl can cause operational problems on the papermachine.
One prior-art method for reducing forming fabric edge curl is disclosed in U.S. Pat. No. 4,941,239 to Fliss, which patent is commonly assigned with the present invention. The method requires the removal of mass from the sheet-forming side of the forming fabric. This has the effect of reducing the ratio between the shrink forces acting on the two sides of the forming fabric in the cross-machine direction and, in turn, reduces the tendency for shrinkage to cause the edges of the fabric to curl. More specifically, the mass is removed by using a fine abrasive medium on the surface of the fabric. While one could remove mass from the entire sheet-forming side of the forming fabric in this manner, it is preferable to so treat only regions lying in a band along each of the two lateral edges thereof, so as not to adversely affect its paper-forming characteristics.
The present invention provides another solution to the problem of forming fabric edge curl.